Known and uncharacterized GPCRs (G-protein coupled receptors) currently constitute major targets for drug action and development, and >30% of all marketed therapeutics act on them (Jacoby et al 2006, Chem Med Chem 1, 760-782). GPCRs usually have seven transmembrane domains. Upon binding of a ligand to an extra-cellular portion or fragment of a GPCR, a signal is transduced within the cell that results in a change in a biological or physiological property or behavior of the cell. GPCRs, along with G-proteins and effectors (intracellular enzymes and channels modulated by G-proteins), are the components of a modular signaling system that connects the state of intra-cellular second messengers to extra-cellular inputs (Pierce et al 2002, Nature Reviews Molecular Cell Biology 3, 639-650). The GPCRs seem to be of critical importance to both the central nervous system and peripheral physiological processes.
The GPCR superfamily is diverse and sequencing of the human genome has revealed >850 genes that encode them (Hopkins and Groom 2002, Nature Reviews Drug Discovery 1, 727-730). There is great diversity within the GPCRs, which is matched by a great variety of ligands that activate them. Known drugs target only 30 members of the GPCR family, mainly biogenic amine receptors. Thus, there is an enormous potential within the pharmaceutical industry to exploit the remaining family members, including the >100 orphan receptors for which no existing ligands have so far been identified (Gilchrist 2004, Expert Opin. Ther. Targets 8, 495-498).
There are ongoing efforts to identify new GPCRs and to deorphanize known GPCRs, which can be used to screen for new agonists and antagonists having potential prophylactic and therapeutical properties (Gilchrist 2004, Expert Opin. Ther. Targets 8, 495-498; Schyler and Horuk 2006, Drug Discovery Today 11, 481-493). Below are examples of GPCRs pertinent to this application, which may serve as targets for novel therapeutic agents.
The Mas receptor is the product of the MAS1 proto-oncogene, which was first isolated based on its tumorigenic activity and later identified as a member of the rhodopsin-like class A GPCR subfamily. It was recently demonstrated that Ang(1-7) is an agonist of Mas and that this peptide is formed by the action of ACE2 (angiotensin converting enzyme 2) on angiotensin I (reviewed in Santos et al. 2007, Current Cardiology Reviews 3, 57-64). While the chronic increase in AngII can induce many deleterious effects on the heart, Ang(1-7) has cardioprotective actions, including vasodilation and antiproliferative activities, which often oppose those of AngII. The effects of Ang(1-7) are associated with lowering blood pressure, prevention of cardiac remodeling and attenuation of renal abnormalities associated with hypertension (Reudelhuber 2006, Hypertension 47, 811-815).
Mas, ACE2 and Ang(1-7) are considered important components of the renin-angiotensin system (RAS), which is a major regulator of cardiovascular homeostasis and hydroelectrolyte balance (Silva et al 2006, Mini-reviews in Medicinal Chemistry 6, 603-609; Santos and Ferreira 2007, Current Opinion in Nephrology and Hypertension 16, 122-128). Disturbances in the RAS system play a pivotal role in the pathogenesis of hypertension and cardiovascular diseases. RAS can be viewed as a system comprising two main axes with opposite actions: the vasoconstrictor/proliferative ACE-AngII-AT1/AT2 axis, and the vasodilator/anti-proliferative ACE2-Ang(1-7)-Mas axis.
Components of the ACE-AngII-AT1/AT2 axis serve as targets for two major types of drugs, the ACE inhibitors (ACE1) and the AT1 receptor blockers (ARBs), which are successful therapeutic strategies against several clinical conditions, including arterial hypertension, left ventricular systolic dysfunction, chronic heart failure, myocardial infarction and diabetic and non-diabetic chronic kidney diseases (Ferrario 2006, Journal of the Renin-Angiotensin-Aldosterone System 7, 3-14). The ACE2-Ang(1-7)-Mas axis is considered as a putatively important target for the development of new drugs to treat cardiovascular and renal diseases (Santos et al. 2007, Current Cardiology Reviews 3, 57-64; Keidar et al 2007, Cardiovascular Research 73, 463-469). The potential therapeutic application of the Mas receptor is indeed supported by the cardioprotective and beneficial effects of its peptide agonist, Ang(1-7), and an orally active nonpeptide agonist, AVE 0991, in several experimental models (Santos and Ferreira 2006, Cardiovascular Drug Reviews 24, 239-246).
The FPRL1 receptor belongs to the FPR (formyl-peptide receptor) related family of GPCRs that also includes FPR and FPRL2 (Le et al 2001, Cytokine and Growth Factor Reviews 12, 91-105). This receptor, also known as the lipoxin A4 receptor, ALXR, binds pleiotropic ligands, i.e. both lipids and peptides, and is expressed primarily by neutrophils, eosinophils and monocytes (Chiang et al 2006, Pharmacological Reviews 58, 463-487). The two prominent types of endogenous FPRL1 ligands, lipoxin A4 (LXA4) and the aspirin-triggered lipoxins (ATLs), and the Annexini protein and its N-terminal derived peptides, have shown anti-inflammatory properties in various experimental animal models (Gavins et al 2005, Prostaglandins, Leukotrienes and Essential Fatty Acids 73, 211-219).
Extensive research has clarified that the mechanism underlying the anti-inflammatory activity gained upon FPRL1 activation by these ligands is achieved by promoting resolution of inflammation—an active and tightly synchronized process, involving counter-regulation of leukocytes (Scannell and Maderna 2006, The Scientific World Journal 6, 1555-1573). Activation of FPRL1 evokes inhibition of polymorphonuclear neutrophils (PMN's) and eosinophils migration and prevents leukocyte-mediated tissue injury. In addition, emigration of monocytes is stimulated upon FPRL1 activation, enabling the clearance of apoptotic cells from the inflammatory site in a nonphlogistic manner. Furthermore, NK cytotoxicity is inhibited which further contributes to downregulation of proinflammatory mediators at the site of inflammation.
Both LXA4 (and its stable analog ATLa) and Ac2-26 (a peptide derived from the N-terminus of AnnexinI) have been extensively studied in various animal disease models of acute and chronic inflammation, such as dermal inflammation, colitis, asthma, and ischemia/reperfusion injury, and were found to be efficacious (Perretti and Gavins 2003, News Physiol. Sci. 18, 60-64; Gewirtz 2005, Current Opinion in Investigational Drugs 6, 1112-1115). These findings indicate that FPRL1 agonists open new avenues and approaches to therapeutic interventions via accelerated resolution of inflammation, and might have a beneficial therapeutic value in various pathological inflammatory conditions, such as ischemia/reperfusion injury, organ transplantation, inflammatory bowel disease, psoriasis, asthma, and arthritis. A stable lipoxin analog is indeed in clinical development for inflammatory bowel disease (Berlex).
The MrgX1 (Mas-related gene X1) receptor, also named SNSR4 (sensory neuron specific receptor 4), has been detected only in the nociceptive sensory neurons of the dorsal root ganglia (Dong et al 2001, Cell 106, 619-632). It is preferentially activated by opioid-related peptides, such as the proenkephalin A-derived peptide, BAM22 (Lembo et al 2002, Nature Neurosci. 5, 201-209). MrgX2 is another member of the Mrg family of GPCRs, which also shows high expression in nociceptive neurons, but also in various other tissues. Various high and low affinity ligands have been identified for MrgX2 (Robas et al 2003, J. Biol. Chem. 278, 44400-44404). The physiological role that these receptors play in vivo is not clear.
Based on their expression in nociceptors, which are neurons that mediate nociceptive transmission of pain, Mrg receptors are believed to play a role in the sensation or modulation of acute pain as well as chronic pain associated with nerve injury or inflammation. The Mrg family, and in particular MrgX1, are thus viewed as promising pharmacological targets for the management of pain (Ahmad and Dray 2004; Current Opinion in Investigational Drugs 5, 67-70; Dray 2003, Current Opinion in Anesthesiology 16, 521-525).
MrgX2 is also activated by several secretagogues, and seems to participate in the activation of human mast cells by such substances Tatemoto et al 2006, Biochem. Biophys. Res. Comm. 349, 1322-1328). As such, MrgX2 might provide a novel therapeutic target for the control of diseases involving mast cell activation. CST, a high affinity ligand of MrgX2, is a neuropeptide involved in sleep regulation and locomotor activity (Robas et al 2003, J. Biol. Chem. 278, 44400-44404). CST also emerged as a potential endogenous immune modulator, and has recently shown potent anti-inflammatory activity in experimental animal models (Gonzalez-Rey and Delgado 2006, Drug News Perspect 19, 393-399). MrgX2 may thus also be involved in sleep regulation, and in inflammation.
Another high affinity ligand of MrgX2 is PAMP-12 (Kamohara et al 2005, Biochem. Biophys. Res. Comm. 330, 1146-1152; Nothacker et al 2005, Eur. J. Pharmacol. 519, 191-193), which derives from proadrenomedullin and like other PAMP peptides has vasophysiological functions that appear to relate to several diseases, such as hypertension, chronic renal failure and congestive heart failure and chronic glomerulonephritis (Kobayashi et al 2003, Hypertension Research 26, S71-S78). Based on this, MrgX2 may also be a target of potential hypotension-regulating drugs.